1. Field of the Invention
The present invention relates to a variable power zoom lens, and more particularly, to an enhanced variable power zoom lens that has a zoom ratio of 15× or even higher and an angle of view of 75 degrees or even wider at the wide-angle end, and that is suitable to compact APS-format single-lens reflex cameras.
2. Description of Related Art Including Information Disclosed Under 37 CFR §§1.97 and 1.98
Some prior art variable power zoom lenses include an APS-format compatible anti-vibration zoom lens that has the foremost or first lens group of positive refractivity, the second lens group of negative refractivity, the third lens group of positive refractivity, and the fourth lens group of positive refractivity disposed in sequence from a position closer to the object to photograph where each pair of the lens groups come closer to or space farther apart from each other for varying the magnification power during zooming.
In such a zoom lens, the third lens group consists of a front subset of lens pieces of positive refractivity and a rear subset of lens pieces of negative refractivity disposed in sequence from a position closer to the object. The rear subset alone is moved in a direction perpendicular to the optical axis to correct an imaging position, and at least one of surfaces of the lens pieces in the rear subset is made aspherical in shape so that positive refractive power is enhanced or negative refractive power is diminished as it goes from the optical axis toward the periphery of the lens piece, in comparison with a spherical surface having paraxial radius of curvature.
In addition, such a zoom lens meets requirements as expressed in the following formulae:3.7<f1/fw<7.0where f1 is a focal length of the first lens group, and fw is the focal length of the zoom lens at the wide-angle end. (See Patent Document 1 listed below.)
Some other prior art variable power zoom lenses include an APS-format compatible zoom lens that has the foremost or first lens group of positive refractivity, the second lens group of negative refractivity, and succeeding lens groups with the one of positive refractivity disposed in sequence from a position closer to the object to photograph where the succeeding lens groups include a lens group IS of negative refractivity moved in a direction induced by orthogonal coordinate components to the optical axis in the Cartesian coordinate system so as to shift an imaging position, and a lens group R_IS of positive refractivity positioned behind the lens group IS and closer to the image plane. For zooming, the lens group IS and the lens group R_IS come closer to or space farther apart from each other, and the zoom lens meets requirements as defined in the following formulae:0.05<|fIS/fT|<0.182.0<f1/fR—IS<4.5where fIS is a focal length of the lens group IS, fR_IS is the focal length of the lens group R_IS, and fT is the focal length of the whole optics of the zoom lens at the telephoto end. (See Patent Document 2 listed below.)